In recent years, the weight of automobile bodies has been reduced to improve fuel economy and to reduce CO2 emission. Steel is used as the material of most parts of an automobile because there is a good balance between the cost, formability, and strength of steel. Steel parts have been reduced in weight by reducing their thickness. On the other hand, a higher joint strength is required to ensure the safety of an automobile body. Studies have been made to ensure the reliability of an automobile body by increasing the tensile strength of base metal in body parts while reducing the thickness of the automobile body parts.
Welding is necessary in assembling an automobile body, and the quality of a weld is directly linked to the reliability of an automobile body. Among welding methods used to assemble an automobile body, resistance spot welding has advantages of low operation cost and high degree of flexibility in welding positions. Welding is performed at several thousand spots in each automobile body. Therefore, resistance spot welding has an important function in ensuring reliability of an automobile body.
As illustrated in FIG. 1, resistance spot welding is a method of obtaining a welded joint by clamping a sheet set 3 of two or more lapped steel sheets (here, two sheets comprising of a lower steel sheet 1 and an upper steel sheet 2) between a pair of upper and lower electrode tips (a lower electrode tip 4 and an upper electrode tip 5), pressing and weld the sheet set so as to melt the steel sheet, and forming a nugget 6 having a necessary size.
The quality of the joint obtained in this way is evaluated in terms of the diameter of the nugget, whether full penetration is achieved, the tensile shear strength (the strength of a joint when a tensile test is performed by applying tension in a shearing direction of the joint), the cross tension strength (the strength of a joint when a tensile test is performed by apply tension in a peeling direction of the joint), the fatigue strength, and the like. Among these, the static strengths, which are typically measured by the tensile shear strength and the cross tension strength, are important as an index of the quality of the joint.
It is known that the tensile shear strength of a spot welded joint tends to increase as the tensile strength of a steel sheet increases. However, the cross tension strength may increase only negligibly as the tensile strength of a steel sheet increases, or may decrease on the contrary. It is considered that this is because the carbon equivalent Ceq of a steel sheet, which is represented by the following equation or the like, increases as the strength of the steel sheet increases, and when steel sheets with a high Ceq value are welded, the hardness of the weld and the heat affected zone increases as the steel sheet is subjected to a heat cycle including rapid heating and rapid cooling, and thereby the toughness of the weld decreases.Ceq=C+ 1/30×Si+ 1/20×Mn+2P+4S(%)
In order to ensure the strength of a welded joint manufactured by resistance spot welding of high strength steel sheets, the welding method may be improved by increasing the number of welds or by increasing the nugget diameter. However, because a larger welding workspace is necessary to increase the number of welds, increase in the number of welds would increase the operation time and decrease the productivity. To increase the nugget diameter, it is necessary to increase the size of electrodes and increase the electrode force applied to the steel sheets to prevent expulsions. These are constrained by the equipment and also have a disadvantage that the characteristics of a base metal may be impaired because the heat affected zone is enlarged.
Therefore, in order to ensure the strength of a welded joint with a nugget diameter that is equal to or smaller than those of existing methods, in-process postheating treatment methods, in which is performed after main current for forming a nugget, have been developed. In particular, a method to temper martensite (hereinafter referred to as “tempering method) is a method in which a weld is temporarily cooled and then reheated. With this method, a nugget is temporarily solidified, transformed to hard martensite, and then reheated, and thereby a nugget and a heat affected zone (HAZ) are softened. As a result, the toughness of the nugget is increased and the stress concentration in the vicinity of the weld is reduced, and thereby increase in the strength of the joint can be realized. A large number of studies have been made on this method.
For example, Patent Literature 1 describes that it is preferable that the product of the square of (It/To) and (Tt/To) is in the range of 0.25 to 0.82, where Tt and It are the weld time and the weld current of in-process tempering treatment, respectively and To and To are the weld time and the weld current of main welding, respectively.
Non Patent Literature 1 describes that static strength is increased by performing in-process tempering treatment on steel sheets having a thickness of 1.05 mm and the total time required for in-process postheating treatment is 0.9 seconds, including a cooling time of 0.4 seconds and a in-process tempering treatment time of 0.5 seconds.
Patent Literature 2 describes that the cross tension strength of high tensile strength steel sheets can be increased by performing in-process treatment with a weld current that is lower than or equal to the main welding current after performing the main welding, and by changing the holding time after the welding in accordance with the thickness of the steel sheets.
Patent Literature 3 describes that the cross tension strength of a joint manufactured by welding steel sheets having a tensile strength in the range of 900 to 1850 MPa can be increased by, after performing main welding, performing welding for 40 to 80 ms with an weld current that is in the range of 70 to 90% of the main welding current, or performing, after a cooling time of 20 ms, welding for 40 to 200 ms with an weld current in the range of 40 to 70% of the main welding current.
In recent years, as described in Patent Literature 4 and Non Patent Literature 2, methods of improving the cross tension strength by performing a certain cooling and then performing welding for about 2 to 4 cycles (40 to 80 ms) have been proposed. Non Patent Literature 2 describes that an effect equivalent to that of a tempering method can be obtained by performing cooling for about 40 cycles (0.8 s) and then performing welding. Patent Literature 4 describes that a specific thermal effect is obtained by applying an weld current near the upper limit at which expulsion occurs, and thereby an effect equivalent to that of a tempering method can be obtained.
In recent years, pulsation methods, which are different from tempering methods and in which cooling and welding are alternately repeated, have been studied. For example, Patent Literature 5 describes that the strength of a joint can be increased in a shorter weld time than tempering methods by, after performing main welding for forming a nugget, holding a sheet set in a non-welding state, performing welding for a short time using an weld current higher than the main weld current, and repeating this cycle for a plurality of times.
There is a problem in that, when welding a sheet set including a thin sheet and two thicker sheets, it is difficult to form a fusion zone between the thin sheet and the thicker sheets. Patent Literature 6 describes that, for a sheet set including three or more such sheets, a sufficient nugget diameter can be formed by performing pulsation welding in which cooling and welding are alternately repeated after having performed main welding.
Patent Literature 7 describes that a nugget can be formed without causing expulsion by, after performing main welding for forming the nugget, performing welding with a lower weld current, performing welding for a short time using an weld current higher than the main weld current, and repeating this cycle for a plurality of times.